One of the major challenges in cochlear implants (CIs) is the large variability in CI patients? speech perception performance. There are currently no objective measures that can be used clinically to predict speech outcomes of CI users who cannot reliably perform tests requiring behavioral responses. Moreover, the lack of understanding of the large variability on CI outcomes is a barrier to customized rehabilitation. Our long-term goal is to understand the neural correlates of the variability in CI speech outcomes and to explore if these neural responses can serve as objective measures to assess and predict CI outcomes. Previous studies have shown that the capability to detect frequency changes of sound measured using psychophysical methods is highly correlated with speech outcomes in CI users. In fact, the ability to detect frequency changes may play an important role in the performance on pitch-related tasks that typically challenging for CI users such as speech perception in noisy environments. The acoustic change complex (ACC) is a type of cortically-generated auditory evoked response evoked by the change in the frequency, amplitude etc. of an acoustic stimulus. Recording the ACC does not require any behavioral response from an individual. The objective in this R15 project is to examine the predictive value of the ACC in CI adult users. In this project, we will determine the correlation between the ACC and behavioral performance of frequency change detection and speech perception when the stimuli are presented without a background noise (Aim 1) and with a background noise (Aim 2), respectively. Adult CI users and normal hearing (NH) controls will have their speech recognition abilities assessed and they will undergo electroencephalographic recordings while they perform the psychoacoustic task of identifying stimuli containing frequency changes. The ACC measures (peak amplitude, latency, and brain activation patterns) will be compared with the behavioral measures of frequency change detection and speech outcomes. The central hypothesis is that the ACC measures are significantly correlated to CI users? behavioral performance in frequency change detection and speech recognition abilities. This project will have significant implications: 1) the results from the proposed study will provide important insights into the large variability in CI patients? speech understanding, 2) the proposed research will help to identify electrophysiological measures that can be used to objectively assess CI benefits. Such objective measures are especially valuable for individuals who cannot reliably perform behavioral tests, and 3) this research may lead to future efforts toward improving the cortical encoding of frequency changes in CI users to maximize CI benefits.